In recent years, services to deliver contents such as movies and music have been provided. Users using such services pay charges to service providers for using the services and enjoy the contents. Contents may be delivered online through the Internet, terrestrial broadcasting, satellite broadcasting, cable television lines, optical lines, telephone lines, or a communication network including a combination thereof, or may be recorded on recording media such as compact disks (CD), flexible disks (FD), digital versatile disks (DVD) and blue-ray discs (BD) to be distributed offline. Some service providers encrypt contents before delivering them so as to permit only authorized users to access the contents. If decryption keys for the contents that are encrypted (encrypted contents) are issued only to authorized users, users who are not authorized and do not have the decryption keys (unauthorized users) cannot decrypt the encrypted contents. However, encryption of contents alone may not be sufficient to counter such threads as follows. For outputting a content (for example, displaying video on a display device) on the user side, a decrypted content is transmitted to an output device such as a display device. Then, a malicious user might take out the content output to the output device (for example, capturing and taking out the video that is displayed on the display device), and re-distribute the content to unauthorized users or disclose the content by uploading it onto a site on the Internet such as a video sharing site. Digital watermarking technologies are known as countermeasures against such threads.
Digital watermarking allows embedding, into contents such as audio, music, video, static images that are converted into digital data, of information such as copyright holder identification information of the contents, copyright holder identification information, content identification information, user identification information, distributor identification information, use conditions of the contents, secret information necessary for using the contents and copy control information in a state where such information cannot be recognized by human perception. For example, if user identification information is embedded as a digital watermark in a content and if the content is leaked, the user who leaked the content can be identified by detecting the user identification information embedded in the leaked content. Therefore, unauthorized users can be deterred from re-distributing contents by adopting the digital watermarking technologies. Moreover, if content identification information is embedded as a digital watermark in a content and if the content is disclosed on an Internet site such as a video sharing site, the content to be deleted therefrom can be identified by detecting the content identification information embedded in the disclosed content. If these processes are to be automatically performed to some extent, it is possible to efficiently find contents to be deleted among a huge number of contents disclosed on Internet sites and to request operators of the Internet sites to delete the contents in early stages. Therefore, contents uploaded by unauthorized users can be efficiently deleted by using the digital watermarking technologies. In addition, usage of the digital watermarking technology can improve copyright protection including usage control and copying control and can promote the secondary usage of the products.
Various techniques have been proposed for digital watermarking schemes. A scheme based on the spread spectrum technology is known as one of such techniques. There has been proposed a technology of embedding watermark information into an original image, by using a signal that has a specific frequency component and that is extracted from the image signal of the original image into which the watermark information is to be embedded. Watermark information refers to information to be embedded as a digital watermark.
If the amount of watermark information is large, the watermark information needs to be divided and be embedded into a content at respective temporal intervals. Dividing and embedding watermark information into a content at respective temporal intervals means, in a case where a symbol string “ABC” is to be embedded as watermark information, for example, such operations as: embedding “A” into an interval α (for example, the first one minute); embedding “B” into an interval β (for example, the next one minute); and embedding “C” into an interval γ (for example, the next one minute after the next). If watermark information is divided and is embedded into a content at respective temporal intervals in this manner, boundaries between the intervals have to be known for detecting the watermark information from the content in which the watermark information is embedded. This is because the beginning of a leaked content cannot be expected to be coincident with the beginning of a certain interval since a content that is re-distributed by an unauthorized user is a part of a distributed content extracted therefrom (for example, an extract of a several minutes containing a climactic scene in a content such as a movie). If an interval including the latter half of the interval α and the former half of the interval β is incorrectly detected for the interval α and detection is made from the incorrect interval, it is likely that incorrect watermark information is detected or the detection result cannot be determined. Thus, there is an issuer that it is necessary to correctly detect from which time point to which time point in a leaked content each symbol is embedded in order to correctly detect watermark information from the leaked content.
As one solution to this problem, features of a content may be used. If the content is of video, one of the features of the content includes a scene change. A scene change is an event that a histogram variation between a previous frame and a next frame at a display time exceeds a certain threshold. A scene change point is a point where a scene change occurs. There has been proposed a technique that embeds a digital watermark in synchronization with scene changes in video so as to correctly detect watermark information even if a time lag occurs.
If the technique just mentioned above is used for dividing and embedding watermark information into a content at respective temporal intervals, the following problem occurs. The problem will be described using an example where a symbol string “ABC” is to be embedded as watermark information. For example, in a case where there are two scene change points in a content to be distributed, it is assumed that time point t0, . . . , t7 each represents time (for example, time elapsed from the beginning of the content) at a boundary between intervals; time t0 corresponds to a first scene change point; and time t4 corresponds to a second scene change points. In this case, starting points of embedding “A” are determined to be coincident with the scene change points t0 and t4. It is assumed that “A” is embedded as watermark information into an interval “t1−t0” from the first scene change point t0 as the starting point to time t1; “B” is embedded as watermark information into an interval “t2−t1”; and “C” is embedded as watermark information into an interval “t3−t2”. Further, “A” is embedded as watermark information into an interval “t5−t4” from the second scene change point t4 as the starting point to time t5; “B” is embedded as watermark information into an interval “t6−t5”; and “C” is embedded as watermark information into an interval “t7−t6”. It is assumed here that an unauthorized user extracted a part starting from a point between time t0 and time t1 and ending at a point between time t4 and time t5 from the content; and leaked the extracted part. In this case, there is only one scene change point, which is t4, contained in the leaked part of the content. In this case, although “A” that is embedded in the interval from time t4 to time t5 can be detected, the rest of the watermark information cannot be detected correctly since information on the other intervals cannot be obtained. If it is assumed that a plurality of scene change points is present from time t0 to time t3 and that an unauthorized user extracted and leaked the content as described above, there is a plurality of scene change points contained in the leaked part of the content. In this case, it is difficult to correctly detect the whole watermark information since it is not known from which scene change point embedding of the watermark information starts. As described above, it is difficult to correctly detect watermark information if the technique mentioned above is used for dividing and embedding watermark information into a content at respective temporal intervals.